MOTHERS who want a quiet night may need to more careful about how they use expressed milk.
The naturally occurring chemicals in human breast milk that cause sleepiness are most concentrated in milk expressed at night, so breast milk pumped and bottled during the day may not help babies to the land of nod if given to them at night.
鈥淚t is a mistake for the mother to express the milk at a certain time and then store it and feed it to the baby at a different time,鈥� says Cristina S谩nchez at the University of Extremadura in Badajoz, Spain.
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The sedatives are nucleotides, which also take part in signalling and metabolic processes in cells. Several have been implicated in the processes of sleep.
The chemicals had already been found in breast milk, the concentrations increasing in the first few weeks after birth, so it seemed likely that they had a developmental function. Now it looks as though they have a more immediately useful role.
S谩nchez and colleagues looked at the concentrations of 5鈥橴MP, 5鈥橝MP and 5鈥橤MP 鈥� the three nucleotides most strongly associated with sleep and sedation 鈥� in the breast milk of 30 healthy mothers who had been breastfeeding for at least three months. Samples of milk were collected before each feed over 24 hours, with between six and eight samples collected per mother.
They found that concentrations of 5鈥橝MP were highest in the evening, while levels of 5鈥橤MP and 5鈥橴MP increased as the night wore on. These sedatives were found at much lower concentrations in milk expressed during the day (Nutritional Neuroscience, ).
S谩nchez suggests that 5鈥橝MP in breast milk might be fuelling the release of the sleep-promoting neurotransmitter GABA, while 5鈥橤MP is involved in the secretion of melatonin, which helps regulate the body鈥檚 natural clock. 5鈥橴MP is known to encourage both REM and non-REM sleep.
S谩nchez and her colleague Javier Cubero have also created a 鈥渘ight-time鈥� milk by adding 5鈥橝MP and 5鈥橴MP to standard formula milk. Infants receiving this milk between 6 pm and 6 am, and normal milk during the day, fell asleep faster and spent longer sleeping than when they drank standard formula milk all the time (Neuroendocrinology Letters, vol 28, p 360).